1. Field of the Invention
The present invention relates to call back services in a communication system.
2. Description of the Related Art
A diverse range of communication systems are in use today enabling communication between two or more entities, such as user equipment and/or other nodes associated with the system. Such systems may comprise, for example, communication of voice, data, and multimedia data.
Communication systems providing wireless communication for user terminals or other nodes are known. An example of a wireless system is a public land mobile network (PLMN). A PLMN is typically a cellular network wherein a base transceiver station (BTS) or similar access entity serves user equipment (UE) such as mobile stations (MS) via a wireless interface. The operation of the apparatus required for the communication is usually controlled by one or more control entities, which themselves may be interconnected. One or more gateway nodes provide for connecting the PLMN to other networks. Examples of other such networks are another cellular network, a public switched telephone network (PSTN) and packet switched data networks such as an IP (Internet Protocol) based network. The communication between the user equipment and the other elements of the communication system are based on an appropriate communications protocol, which defines the “rules” under which communication is handled in the system.
In the current third generation (3G) wireless system, there are defined various servers for the handling of different communication services for mobile users. These include servers that provide call session control functions, known as CSCFs. Control functions may also be provided by entities such as a home subscriber server (HSS) and various application servers (AS). The HSS is typically for permanently storing the user's (subscriber's) profile. For example, in the Release 5 architecture for 3G, as specified by the 3rd Generation Partnership Project (3GPP), these entities can be found located in the IP Multimedia Subsystem (IMS). Call session control functions provides a serving function S-CSCF, an interrogating function, I-CSCF and a proxy function P-CSCF.
IMS user terminals, although using the IMS network (that requires packet-switched connectivity, but not circuit-switched connectivity), usually also implement classical circuit-switched telephony terminals.
The IMS network may sit at the hub of the 3G architecture, supporting an IP based network that handles advanced multimedia services. The 3GPP has chosen Session Initiation Protocol (SIP) as a core session signalling protocol for 3G IP Multimedia Subsystem (IMS) networks. SIP has been developed by the Internet Engineering Task Force (IETF) in RFC 3261 and a number of other RFCs. The 3GPP specification 24.229 describes the IMS network basic operation from an SIP perspective. It should be noted that SIP is a request/response style protocol, in the sense that for every message sent from a source, there is one or more associated responses from the destination confirming receipt of the sent message.
Another protocol under discussion is the TEL Uniform Resource Identifier (URI). The TEL URI was originally specified in RFC 2806, http://www.ietf.org/rfc/rfc2806.txt and it is currently under revision. The TEL URI provides a mechanism to express a telephone number as a URI. Since SIP can carry URIs, the TEL URI can be applied to SIP, so that SIP can transport TEL URIs (telephone numbers). Unfortunately, TEL URIs do not provide SIP routing information, therefore, TEL URIs have to be translated at a some stage into routable SIP URIs.
The translation of TEL URIs into SIP URIs is typically done with an existing translation mechanism known as E.164 number translation (ENUM), which maps an international E.164 number into an SIP URI. ENUM was originally specified in RFC 2916, http://www.ietf.org/rfc/rfc2916.txt and has been recently revised in RFC 3761, http://www.ietf.org/rfc/rfc3761.txt and RFC 3764, http://www.ietf.org/rfc/rfc3764.txt. The mechanism uses the hierarchical Domain Name Server (DNS) system to supply a SIP URI associated to a TEL URI.
Existing wireless communications systems allow a user to roam between networks. According to existing phone number information protocols, there may be differences between the way in which a calling party's number is displayed to a called party, depending on whether the called party is in his home network or in a foreign (visited) network. That is, if a user is roaming to a country different to his home country, calls originated in that country may display the local national format of the calling party's number to the user, whereas calls originated in the user's home country will be displayed in international format to the user. An added complexity is that a national telephone number is valid in the context of a country where the user is roaming. It will be appreciated that each country can provide one or more networks, some or all of which may have roaming agreements with the home network.
FIG. 1A illustrates the problem. A first user equipment UE A represents the calling party and the second user equipment UE B represents the called party. The user equipment UE B is roaming in a visited network and the user equipment UE A is a subscriber of that said visited network or another network in the same country. Where the user of user equipment UE A is calling to user B, the number (calling party number) which is displayed at the user equipment UE B once the CS call is established is represented in national format. The calling party number: 050 555 6666 is illustrated in FIG. 1A.
It is expected that in the future the called party more often needs to establish a callback type of services to the original calling party, in particular for the establishment of services such as SIP/IMS sessions to enrich an existing voice call which has already been established, or to provide SIP/IMS sessions after the call has terminated. To establish a callback, the original called party UE B should issue an INVITE request, as shown in FIG. 1A, but in order for the network to route an SIP session from the user equipment UE B to the user equipment UE A, the network needs to know the SIP URI (Uniform Resource Identifier) of the user A.
An additional problem when circuit-switched calls are complemented with SIP/IMS sessions resides in the fact divergence of the call control in roaming scenarios between both CS calls and SIP/IMS sessions. CS calls follows the visited network control, i.e. an entity in the visited network (e.g. an MSC or MSC server) controls and provides the routing towards the recipient of the call, whereas IMS follows the home control model, where routing towards the recipient is provided by an entity in the home network (e.g. S-CSCF). This divergence originates a routing problem when telephone numbers obtained from the local circuit-switched network are extrapolated and used in the IMS.
The ENUM translation mechanism mentioned above requires as an input the telephone number in international E.164 format. It will be evident that in the situation where user B obtains a national telephone number from the user equipment UE A, the ENUM translation mechanism cannot be utilised when the call control of the user is in his home network, since the home network cannot utilise the national telephone number of some foreign network/country. This is represented by the query marks in the INVITE request in FIG. 1A